In the past, in order to provide a water resistant paperboard it has been common practice to coat the paperboard with petroleum or vegetable based waxes or other coating products. These coatings, particularly the petroleum based products such as paraffin, present a disposal problem because burning them can create air pollution and, when buried in a landfill, they do not readily deteriorate. Accordingly, it is one object of the present invention to provide a moisture resistant paperboard which does not require the application of a coating.
In the process of manufacturing corrugated paperboard usually a strip of paperboard is corrugated by heated, fluted rolls. The protruding tips on one side of the fluted paperboard strip are coated with an adhesive after which a flat sheet of facing paperboard is positioned in contact with these tips. Heat and pressure are applied as the fluted strip and facing board are held together to complete the bond. A double-face paperboard can be made in which the flute tips on both sides of the corrugated strip are coated and the fluted strip is then sandwiched between two paperboard facing sheets.
The selection of an appropriate adhesive is important if the desirable properties of a paperboard product are to be achieved. Perhaps the most commonly used adhesives are the starch-based adhesives which are prepared in various cooking steps with caustic soda and Borax. The type of starch selected is also important and the high amylose starches are traditionally thought to be superior to the lower amylose starches such as pearl starches. To improve the performance of starch adhesives various additives have been employed including cross-linking agents, polyvinyl alcohols, formaldehydes, ketones and acetones. In U.S. Pat. No. 5,093,393 which issued to John A. Faber et al. on Mar. 3, 1992, the use of a controlled amount of polyvinyl alcohol in a pre-mix is described in which the PVA has an intermediate degree of hydrolysis. This pre-mix is used in making a corrugating adhesive comprising starch, alkali base, boric acid, and water. The composition is said to impart an increase in the rate of viscosity build up of the adhesive which increases the bond setting speed. Other prior art patents that describe the use of PVA in starch based adhesives are set forth in the aforementioned Faber et al. patent. In addition, patents such as U.S. Pat. No. 4,814,039 to Willging and U.S. Pat. No. 5,503,668 to Giesfeld et al. also describe PVA's use in a starch based adhesive. Accordingly, it is one object of the invention to provide a novel PVA composition and method of use that provides improved adhesive strength of a starch based adhesive.
While there may be some improvement in moisture resistance or wet strength by the prior art adhesives mentioned above, still the loss of strength under high moisture conditions continues to be a problem. Accordingly, another object of the present invention is to provide an adhesive formulation that provides improved strength in moisture conditions and better performance by corrugated containers.
These objects and others are achieved by the invention described below.